FZN Stands for Five Zero Nine, not Frozen. The 509 FZN Merino base layer collection makes sure of that. This Merino wool blend is high performance and comfortable in all conditions. Comparing our 509 Wool to grandpas scratchy Wool is like comparing your RMK to his 1978 Panther. Our Merino wool is a 20 micron Merino wool natural fiber with incredible moisture management, and Temperature management capabilities.